ABSTRACT (Project 2: Brusseau, Field, Maier, Chorover) Surface-water and groundwater contamination from acid rock drainage is one of the primary environmental concerns for many hardrock mining sites in the USA. Concomitantly, cost-effective treatment of mining-impacted waters to levels protective of human health and the environment is a critical issue for mitigating the risk associated with hardrock mining sites. The overall goals of this project are to investigate the key physical and biogeochemical processes that control migration and attenuation of mine-drainage contaminants in groundwater, and to develop and test innovative methods for remediation of contaminated groundwater at hardrock mining sites. The project incorporates innovative methods to examine the biogeochemical processes that control contaminant behavior, with a focus on the unique properties and conditions inherent to mining sites in the Southwestern US. The project targets the primary groundwater contaminants of concern at these sites, including arsenic, uranium, and sulfate. This project will advance the state of the science regarding the transport and attenuation of metal(loids) in the subsurface. This will improve the development and parameterization of advanced biogeochemical models, and enhance the accuracy of risk assessments. The project outcomes will also improve the design and implementation of groundwater remediation efforts. Specifically, in-situ biosequestration is one of the very few options that exist for remediation of the large, deep groundwater contaminant plumes that form at hardrock mining sites in the Southwestern US. This method has great potential, but several critical questions exist as barriers to widespread acceptance and adoption. This project will answer these questions. It is anticipated that the application of project outcomes will produce significant cost savings for the clean-up of the nation's inventory of mine waste sites.